Coating for gas sensors

ABSTRACT

A method for making a sensor is disclosed. The method comprises: disposing an electrolyte between a first side of sensing electrode and a first side of reference electrode, disposing a first side of a protective layer adjacent to said a second side of said sensing electrode, applying a mixture of a metal oxide, a fugitive material, and a solvent to a second side of the protective layer, and calcining the applied mixture to form said a protective coating on the second side of the protective layer.

TECHNICAL FIELD

The present disclosure relates to gas sensors, and particularly tosensors with a porous protective layer for protection of the sensorelectrode from poisoning.

BACKGROUND

The automotive industry has used exhaust gas sensors in automotivevehicles for many years to sense the composition of exhaust gases,namely, oxygen. For example, a sensor is used to determine the exhaustgas content for alteration and optimization of the air to fuel ratio forcombustion.

One type of sensor uses an ionically conductive solid electrolytebetween porous electrodes. For oxygen, solid electrolyte sensors areused to measure oxygen activity differences between an unknown gassample and a known gas sample. In the use of a sensor for automotiveexhaust, the unknown gas is exhaust and the known gas, (i.e., referencegas), is usually atmospheric air because the oxygen content in air isrelatively constant and readily accessible. This type of sensor is basedon an electrochemical galvanic cell operating in a potentiometric modeto detect the relative amounts of oxygen present in an automobileengine's exhaust. When opposite surfaces of this galvanic cell areexposed to different oxygen partial pressures, an electromotive force(“emf”) is developed between the electrodes according to the Nernstequation.

With the Nernst principle, chemical energy is converted intoelectromotive force. A gas sensor based upon this principle typicallyconsists of an ionically conductive solid electrolyte material, a porouselectrode with a porous protective overcoat exposed to exhaust gases(“exhaust gas electrode”), and a porous electrode exposed to a knowngas' partial pressure (“reference electrode”). Sensors typically used inautomotive applications use a yttria stabilized zirconia basedelectrochemical galvanic cell with porous platinum electrodes, operatingin potentiometric mode, to detect the relative amounts of a particulargas, such as oxygen for example, that is present in an automobileengine's exhaust. Also, a typical sensor has a ceramic heater attachedto help maintain the sensor's ionic conductivity. When opposite surfacesof the galvanic cell are exposed to different oxygen partial pressures,an electromotive force is developed between the electrodes on theopposite surfaces of the zirconia wall, according to the Nernstequation:$E = {\left( \frac{{- R}\quad T}{4F} \right)\quad \ln \quad \left( \frac{P_{O_{2}}^{ref}}{P_{O_{2}}} \right)}$

where

E=electromotive force

R=universal gas constant

F=Faraday constant

T=absolute temperature of the gas

P_(O) ₂ ^(ref)=oxygen partial pressure of the reference gas

P_(O) ₂ =oxygen partial pressure of the exhaust gas

Due to the large difference in oxygen partial pressure between fuel richand fuel lean exhaust conditions, the electromotive force (emf) changessharply at the stoichiometric point, giving rise to the characteristicswitching behavior of these sensors. Consequently, these potentiometricoxygen sensors indicate qualitatively whether the engine is operatingfuel-rich or fuel-lean, conditions without quantifying the actualair-to-fuel ratio of the exhaust mixture.

In a conventional sensor, the sensor comprises a first electrode capableof sensing an exhaust gas and a second electrode capable of sensing areference gas with an ionically conductive solid electrolyte disposedtherebetween. High temperatures and materials such as silicon, lead andthe like, present in engine components, can poison or otherwise damagethe sensing electrode. In order to prevent poisoning/damage to thesensing electrode, a protective layer made of spinel or the like, hasconventionally been applied to the sensing electrode.

The protective layer is designed to allow for the electrodes to sensethe particular gas without inhibiting the performance of the sensor. Athick layer (or multiple layers) of protective coating more effectivelyinhibits the transmission of the poisoning materials, but at the expenseof a decrease in the efficiency of the sensor. Furthermore, theprotective layer itself can become clogged, inhibiting passage ofexhaust gases for sensing. One conventional poison resistance techniquecomprises applying multiple layers of a heat resistant metal oxide tothe electrode to form a protective layer. However, the multiple layershave a tendency to change the performance of the sensor and only providelimited poison protection.

Accordingly, there exists a need in the art for improved protectivecoatings for gas sensors.

SUMMARY

The drawbacks and disadvantages of the prior art are overcome by thecoating for a gas sensor and method for making the same. The method formaking a sensor comprises: disposing an electrolyte between a first sideof sensing electrode and a first side of reference electrode, disposinga first side of a protective layer adjacent to said a second side ofsaid sensing electrode, applying a mixture of a metal oxide, a fugitivematerial, and a solvent to a second side of the protective layer, andcalcining the applied mixture to form said a protective coating on thesecond side of the protective layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the figures, which are meant to be exemplary, notlimiting, and wherein like elements are numbered alike in severalfigures.

FIG. 1 is an expanded isometric view of one embodiment of an oxygensensor.

FIG. 2 is a graph showing low calcium rapid age test (RAT) durability at260° C. for various sensors with hours of RAT exposure time on the Xaxis (hours) and rich to lean response time on the Y axis inmilliseconds (ms).

FIG. 3 is graph showing the siloxane poisoning at 400° C. with hours ofsiloxane exposure time on the X axis (hours) and rich to lean responsetime on the Y axis in ms.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A protective coating for gas sensors, in particular oxygen sensors, isformed from a composition comprising a metal oxide and a fugitivematerial. Although described in connection with an oxygen sensor, it isto be understood that the protective coating can be employed with anytype of sensor such as a nitrogen oxide sensor, hydrogen sensor,hydrocarbon sensor, or the like. Furthermore, while oxygen is thereference gas used in the description disclosed herein, it should beunderstood that other gases could be employed as a reference gas.

Referring to FIG. 1, the sensor element 10 is illustrated. The exhaustgas (or outer) electrode 20 and the reference gas (or inner) electrode22 are disposed on opposite sides of, and adjacent to, an electrolytelayer 30 creating an electrochemical cell (20/30/22). On the side of theexhaust gas electrode 20 opposite solid electrolyte 30 is an optionalprotective insulating layer 40 with a porous section 32 that enablesfluid communication between the exhaust gas electrode 20 and the exhaustgas. A protective coating 31 can be disposed over the porous section 32.The electrolyte 30 and the porous section 32 can be disposed adjacentto, or as inserts within, layers 40, 42, respectively. Meanwhile,disposed on a side of the reference electrode 22 opposite electrolytelayer 30 is a heater 60. Typically disposed between the reference gaselectrode 22 and the heater 60, as well as on a side of the heater 60opposite the reference gas electrode 22, are one or more insulatinglayers 50, 52.

In addition to the above sensor components, conventional components canbe employed, including but not limited to lead gettering layer(s),leads, contact pads, ground plane(s), support layer(s), additionalelectrochemical cell(s), and the like. The leads (24), which supplycurrent to the heater and electrodes, are typically formed on the samelayer as the heater/electrode to which they are in electricalcommunication and extend from the heater/electrode to the terminal endof the gas sensor where they are in electrical communication with thecorresponding via (not shown) and appropriate contact pads (not shown).

Insulating layers 50, 52, and protective layer 40, provide structuralintegrity (e.g., protect various portions of the gas sensor fromabrasion and/or vibration, and the like, and provide physical strengthto the sensor), and physically separate and electrically isolate variouscomponents. The insulating layer(s), which can be formed using ceramictape casting methods or other methods such as plasma spray depositiontechniques, screen printing, stenciling and others conventionally usedin the art, can each be up to about 200 microns thick or so, with athickness of about 50 microns to about 200 microns preferred. Since thematerials employed in the manufacture of gas sensors preferably comprisesubstantially similar coefficients of thermal expansion, shrinkagecharacteristics, and chemical compatibility in order to minimize, if noteliminate, delamination and other processing problems, the particularmaterial, alloy or mixture chosen for the insulating and protectivelayers is dependent upon the specific electrolyte employed. Typicallythese insulating layers comprise a dielectric material such as alumina,and the like.

Disposed between the insulating layers 50, 52, is a heater 60 that isemployed to maintain the sensor element at the desired operatingtemperature. Heater 60 can be any conventional heater capable ofmaintaining the sensor end at a sufficient temperature to facilitate thevarious electrochemical reactions therein. The heater 60, which istypically platinum, aluminum, palladium, and the like, as well asoxides, mixtures, and alloys comprising at least one of the foregoingmetals, or any other conventional heater, is generally screen printed orotherwise disposed onto a substrate to a thickness of about 5 microns toabout 50 microns.

Disposed on an opposite side of insulating layer 50 as heater 60 is theelectrolyte 30. The electrolyte 30 can be solid or porous, can comprisethe entire layer or a portion thereof, can be any material that iscapable of permitting the electrochemical transfer of oxygen ions,should have an ionic/total conductivity ratio of approximately unity,and should be compatible with the environment in which the gas sensorwill be utilized (e.g., up to about 1,000° C.). Possible electrolytematerials can comprise any material conventionally employed as sensorelectrolytes, including, but not limited to, zirconia which mayoptionally be stabilized with calcium, barium, yttrium, magnesium,aluminum, lanthanum, cesium, gadolinium, and the like, as well ascombinations comprising at least one of the foregoing materials. Forexample, the electrolyte can be alumina and/or yttrium stabilizedzirconia. Typically, the electrolyte, which can be formed via manyconventional processes (e.g., die pressing, roll compaction, stencilingand screen printing, tape casting techniques, and the like), has athickness of up to about 500 microns or so, with a thickness of about 25microns to about 500 microns preferred, and a thickness of about 50microns to about 200 microns especially preferred.

It should be noted that the electrolyte layer 30 and porous section 42can comprise an entire layer or a portion thereof; e.g., they can formthe layer (i.e., 42 and 40, respectively), be attached to the layer(porous section/electrolyte abutting dielectric material), or disposedin an opening in the layer (porous section/electrolyte can be an insertin an opening in a dielectric material layer). The latter arrangementeliminates the use of excess electrolyte and protective material, andreduces the size of gas sensor by eliminating layers. Any shape can beused for the electrolyte and porous section, with the size and geometryof the various inserts, and therefore the corresponding openings, beingdependent upon the desired size and geometry of the adjacent electrodes.It is preferred that the openings, inserts, and electrodes have asubstantially compatible geometry such that sufficient exhaust gasaccess to the electrode(s) is enabled and sufficient ionic transferthrough the electrolyte is established.

The electrodes 20, 22, are disposed in ionic contact with theelectrolyte layer 30. Conventional electrodes can comprise any catalystcapable of ionizing oxygen, including, but not limited to, materialssuch as platinum, palladium, osmium, rhodium, iridium, gold, ruthenium,zirconium, yttrium, cerium, calcium, aluminum, silicon, and the like,and oxides, mixtures, and alloys comprising at least one of theforegoing catalysts. As with the electrolyte, the electrodes 20, 22 canbe formed using conventional techniques. Some possible techniquesinclude sputtering, painting, chemical vapor deposition, screenprinting, and stenciling, among others. If a co-firing process isemployed for the formation of the sensor, screen printing the electrodesonto appropriate tapes is preferred due to simplicity, economy, andcompatibility with the co-fired process. Electrode leads (not shown) andvias (not shown) in the insulating and/or electrolyte layers aretypically formed simultaneously with electrodes.

Following the formation of the sensing element 10, a protective coating31 can be applied to the sensing element 10. This protective coating,which may optionally coat a portion of or all of substrate layer 40and/or support layer 52, is formed from a composition comprising a metaloxide and a fugitive material. Possible metal oxides can includezirconia, alumina, magnesia, titania, and the like, as well as mixtures,alloys, and combinations comprising at least one of the foregoing metaloxides, with a coating comprising alpha alumina, gamma alumina, or deltaalumina, as well as combinations comprising at least one of thesealuminas preferred.

As used herein, “fugitive material” means a material that will occupyspace until the electrode is fired, thus leaving porosity in thecoating. Suitable fugitive materials are accordingly those which willrelease at firing temperatures, and include, but are not limited to,carbon based materials, such as carbon black, graphite, non-dissolvedorganics, and the like, as well as combinations comprising at least oneof the foregoing materials. Preferably, carbon black is used havingparticle sizes of about 0.02 microns (μm) to about 0.2 μm.

The amount of metal oxide and fugitive material used to form theprotective coating 31, as well as the characteristics of thosematerials, is based upon the desired coating characteristics. Theprotective coating 31 preferably has a sufficient porosity with a smallenough pore size to enable the passage of exhaust gases while inhibitingpassage of poisoning particulates. The porosity can be up to about 20%,with about 2% to about 15% preferred, and about 5% to about 12%especially preferred. Meanwhile, a pore size of less than about 25microns (μm), with less than about 10 μm preferred and about 1 μm toabout 2 μm is typically preferred.

As with the pore size and porosity, the thickness of the protectivecoating 31 is based upon the ability to filter out poisoningparticulates while allowing passage of the exhaust gases to be sensed.Although a multi-layered coating can be employed, the protective coatingis preferably a single layer having an overall thickness of up to orexceeding about 200 μm, with a thickness of about 120 μm to about 160 μmpreferred.

Meanwhile, the composition of the unfired protective coating 31 can beup to about 98 weight percent (wt. %) first material (comprising metaloxide), with up to about 10 wt. % fugitive material; with about 93 wt. %to about 97 wt. % first material and about 3 wt. % to about 7 wt. %fugitive material preferred; and about 94 wt. % to about 96 wt. % firstmaterial and about 4 wt. % to about 6 wt. % fugitive material especiallypreferred; based upon the total weight of fugitive material and firstmaterial. In one embodiment, the metal oxide comprises a mixture ofgamma alumina and alpha alumina. Generally, the first material comprisesup to about 30 wt % gamma alumina and up to about 80 wt. % alpha aluminacan be employed and optionally up to about 10 wt % aluminum nitrate;with about 25 wt. % to about 75 wt. % gamma alumina, about 25 wt. % toabout 75 wt. % alpha alumina, and optionally up to about 5 wt. %aluminum nitrate preferred; with about 43.5 wt. % to about 54.5 wt. %gamma alumina, about 43.5 wt. % to about 54.5 wt. % alpha alumina, andabout 1 wt. % to about 3 wt. % aluminum nitrate especially preferred.Preferably, the gamma alumina has an agglomerate size of up to about 25μm or so, with about 6 μm to about 34 μm preferred, while the alphaalumina preferably has a particle size of up to about 1 μm, with about0.3 μm to about 0.5 μm especially preferred.

Although the protective coating 31 can be applied to the porousprotective layer in any conventional fashion using techniques such asimbibing, spraying, spray coating, painting, dipping, spin coating,vapor deposition, and the like, dipping is especially preferred. Forexample, a solution, suspension, ink, paste, slurry, or the like isprepared by mixing the metal oxide(s) with a sufficient amount of afugitive material, such as carbon black, in a sufficient amount of asolvent to attain the desired viscosity mixture. Some possible solventsinclude water, nitric acid, benzoic acid, acetic acid, citric acid, andthe like, as well as a combination comprising at least one of theforegoing solvents. Once the slurry is prepared, the slurry can then beapplied to the desired area of the sensor. Typically the protectivecoating 31 is applied to the protective layer 32 and optionally to thesubstrate layer 40 and/or the support layer 52. (see FIG. 1)

Once the slurry has been applied to the sensor, it is optionally driedat temperatures up to about 100° C. for up to about 1 hour. The driedsensor is then calcined for up to about 10 hour, with less than 5 hourspreferred and about 10 minutes to 60 minutes especially preferred, at atemperature sufficient to burn off the fugitive material. Preferably,calcination is completed at temperatures up to about 1,000° C., withabout 500° C. to about 800° C. preferred, and with about 550° C. toabout 650° C. especially preferred.

The following example is provided to further illustrate the coating fora gas sensor and is not intended to limit the scope thereof. Thefollowing example was used to prepare an exhaust sensor having aplatinum electrode, yttria doped zirconia electrolyte, alumina supportlayers, an alumina protective layer, and a protective coating.

An electrolyte was disposed in an alumina support between two aluminasupports with a platinum electrode screen printed on each support suchthat the electrodes were in intimate contact with the electrolyte.Electrical leads were disposed across the supports from the electrodesto contacts (vias) disposed at an end of the sensor opposite theelectrodes. A protective layer, also disposed in an alumina support, wasthen oriented in physical contact with the sensing electrode, while thereference electrode was disposed in contact with a series of aluminasupport layers, with a heater disposed between the last two supportlayers.

A slurry was then prepared by mixing 4,900 grams (g) of gamma alumina,4,900 g of alpha alumina, 200 g of aluminum nitrate and 490 g of carbonblack with water. The sensor was dipped in the slurry and dried at 60°C. for about 10 minutes. The sensor was then calcined at 650° C. forabout 1.5 hours.

FIGS. 2 and 3 graphically illustrate the low calcium rapid age and thesiloxane poisoning tests, respectively. As can be seen from FIG. 2, thelow density protective coating sensor maintained a rich to lean time(RLT), (under conditions of 260° C., 0.5 hertz (Hz), and an air to fuelratio of +/−0.3 from stoichiometry) of less than about 110 milliseconds(ms) (lines 202) for 400 hours of exposure to the high temperaturecycling of the RAT test.

FIG. 3 illustrates that the sensor prepared in accordance with the aboveexample maintained a substantially better rich to lean response time forthe entire 100 hours as compared to the comparative sensors also tested.Sensors prepared in accordance with the above example maintained a richto lean response time better than about 125 milliseconds (ms) (line 60)and many of these sensors maintained a rich to lean response time betterthan about 90 ms (line 62) for the entire 100 hours. By comparison, theDenso (line 66) sensor with a multi-layer protective coating and the OSS(line 64) sensor had significantly longer rich to lean response timesafter 100 hours of siloxane exposure.

This sensor has a protective coating with lower density and demonstratesbetter resistance to poisoning and improved durability. The coatingapplied is able to resist sensor deactivation as vehicles age because ofthe rough texture of the coating. While smooth, flat coatings are easyto degrade due to the “glassy” zinc phosphate deposition, this coatinghas a superior resistance to diffusion limitation than any othercoating. The sensor exhibits RLT of less than about 130 ms for over 100hours with siloxane poisoning (1.56 ml/gal); over an order of magnitudeimprovement over the prior art. Additionally, a RLT of less than about110 ms for over about 400 hours was achieved in a calcium rapid agetest. It is believed that although conventional sensors fail at about1,000 hours of actual use, this sensor will resist sensor deactivationfor greater than about 2,000 hours with up to and exceeding about 4,000hours feasible. Furthermore, the process to manufacture such a sensordoes not require additional processing steps or time.

While preferred embodiments have been shown and described, variousmodifications and substitutions may be made thereto without departingfrom the spirit and scope of the invention, including the use of thegeometries taught herein in other conventional sensors. Accordingly, itis to be understood that the apparatus and method have been described byway of illustration only, and such illustrations and embodiments as havebeen disclosed herein are not to be construed as limiting to the claims.

What is claimed is:
 1. A method for making a sensor, comprising:disposing an electrolyte between a first side of sensing electrode and afirst side of reference electrode, wherein said sensing electrode has afirst electrical lead in electrical communication with said sensingelectrode, and wherein said reference electrode has a second electricallead in electrical communication with said reference electrode disposinga first side of a protective layer adjacent to a second side of saidsensing electrode; applying a mixture of a metal oxide, a solvent, and afugitive material selected from the group consisting of carbon black,graphite, non-dissolved organics and combinations comprising at leastone of the foregoing materials, to a second side of said protectivelayer; and calcining said applied mixture to form a protective coatingon said second side of said protective layer; wherein said protectivecoating has a thickness of greater than about 120 μm.
 2. The method ofclaim 1, wherein the mixture comprises at least 94 wt. % metal oxide andat least 3 wt. % fugitive material based upon the total weight of thefugitive material and the metal oxide.
 3. The method of claim 2, whereinthe mixture comprises about 93 wt. % to about 97 wt. % metal oxide andabout 3 wt. % to about 7 wt. % fugitive material based upon the totalweight of said fugitive material and said metal oxide.
 4. The method ofclaim 2, wherein the mixture comprises about 94 wt. % to about 96 wt. %metal oxide and about 4 wt. % to about 6 wt. % fugitive material basedupon the total weight of said fugitive material and said metal oxide. 5.The method of claim 1, wherein said protective coating has a thicknessof about 120 μm to about 200 μm.
 6. The method of claim 1, wherein saidsensor is calcined at a temperature of at least 500° C. for at least 10minutes.
 7. The method of claim 6, wherein said sensor is calcined at atemperature of about 500° C. to about 800° C. for about 10 minutes toabout 60 minutes.
 8. The method of claim 6, wherein said sensor iscalcined at a temperature of about 550° C. to about 650° C. for about 10minutes to about 60 minutes.
 9. The method of claim 1, wherein saidfugitive material includes said carbon black, and said carbon black hasparticle sizes of about 0.02 μm to about 0.2 μm.
 10. The method of claim1, wherein said protective coating has a porosity of about 2% to about15%.
 11. The method of claim 1, wherein said metal oxide is selectedfrom the group consisting of alpha alumina, gamma alumina, deltaalumina, and combinations comprising at least one of the foregoing metaloxides.
 12. The method of claim 1, further comprising burning off saidfugitive material.
 13. The method of claim 1, wherein the mixturecomprises about 93 wt. % to about 98 wt % metal oxide and about 2 wt. %to about 7 wt % fugitive material based upon the total weight of thefugitive material and the metal oxide.
 14. A method for making a sensor,comprising: disposing an electrolyte between a first side of sensingelectrode and a first side of reference electrode, wherein said sensingelectrode has a first electrical lead in electrical communication withsaid sensing electrode, and wherein said reference electrode has asecond electrical lead in electrical communication with said referenceelectrode; disposing a first side of a protective layer adjacent to asecond side of said sensing electrode; applying a mixture of a metaloxide, a solvent, and a fugitive material selected from the groupconsisting of carbon black, graphite, non-dissolved organics andcombinations comprising at least one of the foregoing materials, to asecond side of said protective layer; and calcining said applied mixtureto form a protective coating on said second side of said protectivelayer; wherein said protective coating has a thickness of about 120 μmto about 160 μm.
 15. A method for making a sensor, comprising: disposingan electrolyte between a first side of sensing electrode and a firstside of reference electrode, wherein said sensing electrode has a firstelectrical lead in electrical communication with said sensing electrode,and wherein said reference electrode has a second electrical lead inelectrical communication with said reference electrode; disposing afirst side of a protective layer adjacent to a second side of saidsensing electrode. applying a mixture of a metal oxide, a fugitivematerial, and a solvent to a second side of said protective layer; andcalcining said applied mixture to form a protective coating on saidsecond side of said protective layer, wherein said protective coatinghas a thickness of about 120 to about 160 micrometers.
 16. A method formaking a sensor, comprising: disposing an electrolyte between a firstside of sensing electrode and a first side of reference electrode,wherein said sensing electrode has a first electrical lead in electricalcommunication with said sensing electrode, and wherein said referenceelectrode has a second electrical lead in electrical communication withsaid reference electrode; disposing a first side of a protective layeradjacent to a second side of said sensing electrode; applying a mixtureof a metal oxide, a solvent, and a fugitive material selected from thegroup consisting of carbon black, graphite, non-dissolved organics andcombinations comprising at least one of the foregoing materials, to asecond side of said protective layer; and calcining said applied mixtureto form a protective coating on said second side of said protectivelayer; wherein said protective coating has a thickness of greater thanabout 200 μm.
 17. A method for making a sensor, comprising: disposing anelectrolyte between a first side of sensing electrode and a first sideof reference electrode, wherein said sensing electrode has a firstelectrical lead in electrical communication with said sensing electrode,and wherein said reference electrode has a second electrical lead inelectrical communication with said reference electrode; disposing afirst side of a protective layer adjacent to a second side of saidsensing electrode; applying a mixture of a solvent, a fugitive material,and a first material to a second side of said protective layer, whereinsaid first material consists essentially of about 25 wt. % to about 75wt. % gamma alumina, about 25 wt. % to about 75 wt. % alpha alumina, andup to about 5 wt % aluminum nitrate; and calcining said applied mixtureto form a protective coating on said second side of said protectivelayer.
 18. The method of claim 17, wherein said fugitive material isselected from the group consisting of carbon black, graphite,non-dissolved organics and combinations comprising at least one of theforegoing materials.
 19. The method of claim 17, wherein said firstmaterial consists essentially of about 43.5 wt. % to about 54.5 wt. %gamma alumina, about 43.5 wt. % to about 54.5 wt. % alpha alumina, andabout 1 wt. % to about 3 wt. % of aluminum nitrate.
 20. The method ofclaim 17, wherein said protective coating has a thickness of greaterthan about 120 μm.
 21. The method of claim 20, wherein said protectivecoating has a thickness of about 120 to about 160 micrometers.
 22. Themethod of claim 17, wherein said first material consists essentially ofabout 25 wt. % to about 75 wt. % gamma alumina, about 25 wt. % to about75 wt. % alpha alumina, and about 1 wt. % to about 5 wt % aluminumnitrate.